The present invention relates to an electrochemical sensor operable at high temperature and/or high pressure conditions.
Quantitative measurements and evaluation of wellbore fluids are important aspects of determining the quality and economic value of the well. More recently, studies aiming to characterize contents of corrosion-causing components such as H2S, CO2, pH and scale-forming ion species have been reported. This has been driven largely by the need to come up with effective mitigation of the severe impact of corrosion to both oil field operations and investments.
Among the various techniques identified to measure the corrosion contents of wellbore fluids is electrochemical sensing. However, to date, research on high pressure and high temperature electrochemistry has been limited by factors such as corrosion, thermoelectric effects and the decomposition of most polymeric materials. As a result, there still is a great need for thermodynamic data about hydrothermal solutions at high pressure and high temperature conditions such as that of a wellbore. Commercially available press fitted electrodes are limited to use mostly at ambient temperature and pressure conditions due to the eventual penetration of the electrolyte or sensing solution in addition to the wellbore fluids into the small gap found between the electrode materials and the main body.
Earlier designs of sensors specifically for downhole well applications have been reported in Great Britain patent application publication numbers GB2,397,651A, GB2,404,252A and GB2,409,902A and international patent publication numbers WO2004011929 and WO 2004063743.
However, it has recently been confirmed through experimental evaluations that the previously suggested electrode manufacturing methods have problems in applications in extreme wellbore conditions.
Designs that involve press-fitting and/or simple metal mounting procedures on top of electrical pin connectors have failed to obtain the much needed sensor stability and robustness, especially for high temperature and high pressure conditions. Persistent creeping problems, where the liquid electrolyte penetrates into polymer sealant down to the electrical pin sections, have been consistently encountered. This mechanical design failure results in increased background response, or unwanted peak signals related to metal stripping, thus rendering the sensing technique useless, especially at high pressure conditions.